FIG. 1 illustrates one example of a typical power supply system 100 used for plasma processing. The power supply system 100 includes a DC power supply 102 providing DC power to a switching circuit 104 that converts the DC power into pulsed DC and provides the pulsed DC to a plasma load 106. When switching, the potential between nodes C and D passes through a potential of zero, and the plasma can be extinguished or can dim to an extent that it becomes highly resistive and acts like an unfluxed inductor or an open circuit for a short time after this transition. Immediately after this transition, the DC power supply 102 continues to provide power to the switching circuit 104, but most of that power can no longer be delivered to the plasma load 106. Instead, the power predominantly passes through the switching circuit 104 potentially damaging the switching circuit 104.
A snubber 108 can be used to mitigate damage to the switching circuit 104 by absorbing power from the DC power supply 102 during the period after the switching circuit 104 transitions through 0 V. However, existing snubbers are typically dissipative snubbers and/or dissipate significant power.
Additional challenges to known power supply systems include slow processing throughput and further inefficiencies from power dissipation. For instance, and as seen in FIG. 2A, while voltage between nodes C and D can switch with negligible ramp time, current ramps at a much slower pace thus providing an average power that is significantly lower than the power output from the DC power supply 102. This leads to longer processing periods and decreased throughput, since many processes can only end when a predetermined total power has been delivered.
There is also a desire to increase DC pulse frequency provided to the plasma load 106 since this reduces arcing. However, the above-noted problems become more acute at higher frequencies, as illustrated in FIG. 2B. Furthermore, since each pulse is shorter at higher frequency, the current at high frequency may end up larger (in a power-regulated system) than at lower frequency. Since power dissipation is proportional to I2, these larger currents lead to larger power losses. Additionally, switching losses, which are proportional to the current at the moment of switching, are accentuated at higher frequencies since switching current is larger.
FIG. 28 illustrates a unipolar DC plasma processing system 2800 well known to those of skill in the art. The system 2800 includes a DC power source 2802, a sputtering cathode 2804, an anode 2806, and a substrate 2808. These elements can reside wholly or partially within a plasma sputtering chamber 2810. The sputtering cathode 2804 can act as an electrode, but can also be made of sputtering target material that is sputtered onto the substrate when a voltage is applied from the anode to the cathode. The anode 2806 can be a discrete electrode within the plasma sputtering chamber 2810, can in part comprise the substrate 2808 or a substrate holder (e.g., a wafer chuck), can in part comprise an inner surface of the plasma sputtering chamber 2810, or any combination of the above.
This system 2800 can cause unwanted arcing when a dielectric layer forms on the cathode. The presence of the dielectric within the electric field between the sputtering cathode 2804 and the anode 2806 can cause charge to buildup on the outside of the dielectric leading to voltages that cause arcing. There is thus a need for a plasma sputtering system that reduces or avoids charge buildup on the sputtering cathode 2804.